1. Technical Field of the Invention
The present invention relates to an electromechanical device with a deformable element. This type of device may constitute a microswitch particularly suitable for switching elements of an electronic, electrical or optical circuit.
Such a microswitch may comprise a microelectromechanical systems (MEMS) having two different states in order to open or close a circuit and thus actuate or deactuate the operation of an electronic, optical or other device. MEMS are widely used in applications such as telecommunications, radiofrequency communications, portable electronics, commercial, industrial or aerospace electronics, and also in other fields.
2. Description of Related Art
MEMS with deformable elements generally comprise a deformable element in the form of a beam, which is attached, via only one end or by opposed ends, to a substrate and makes it possible to achieve switching between a first stable position and a second stable position by a thermal bimetallic effect, or by electromagnetic and/or electrostatic actuation.
FIGS. 1(a) to 1(c) show a known construction of a microsystem according to European Patent EP 1,220,256, the disclosure of which is hereby incorporated by reference.
This microsystem is produced on a substrate 1. The substrate 1 supports separate conducting elements 50 that are simply separated by a small gap plumb with a deformable element having the form of a beam 10. The beam 10 can deform in a cavity provided in the substrate 1. The beam is provided, on the side of the cavity, with a contact element 40 capable of ensuring electrical continuity between the separate conducting elements 50 when the beam 10 bends into the cavity. The beam 10 supports two resistive elements 21 and 22 located near the ends of the beam and having a thermal expansion coefficient different from that of the beam 10. The elements 21 and 22 form switch control means for switching the beam. Electrostatic retention electrodes are also placed in pairs facing each other, namely the pair of electrodes 15 and 55 on one side and the pair of electrodes 16 and 56 on the other. The electrodes 15 and 16 are supported by the beam 10. They may also be included in the beam. The electrodes 55 and 56 are placed in the bottom of the cavity, on the substrate 1.
FIG. 1(a) shows the microelectromechanical system in the deactivated state, since the contact element 40 does not ensure electrical continuity between the separate conducting elements 50.
When an electrical control current flows directly in the resistive elements 21 and 22 or in electrodes 31 and 32 included in the beam 10 beneath the elements 21 and 22 respectively, the heat supply that results therefrom causes the beam to bend, by the bimetallic effect, towards the bottom of the cavity. The contact element 40 then bears on the separate conducting elements 50 and ensures electrical continuity. This is shown in FIG. 1(b). The microelectromechanical system is then in the actuated state.
The electrodes 15 and 55 on one side and 16 and 56 on the other are then separated by a minimum distance and ensure, by the application of suitable voltages, the electrostatic retention of the bent beam when the electrical current ceases to flow in the resistive elements 21 and 22, or the electrodes 31 and 32, as shown in FIG. 1(c). The removal of the electrostatic retention voltages allows the beam to resume its rest position. The microelectromechanical system then returns to the deactuated state (FIG. 1(a)).
However, this construction has drawbacks as regards ensuring reliability of the contact when faced with wear owing to a very high number of cycles (greater than 109) as in certain types of application. Deterioration of the contact may result in capacitive transmissions between the separate conducting elements in the deactuated position. The use of protuberances on the contacts is not a satisfactory solution because of the difficulties associated with positioning them.
Another problem associated with this embodiment is the electrical voltage needed for retention in the closed position. The lowest possible electrical consumption constitutes in fact a common constraint in all types of microelectromechanical systems, either as regards autonomy in the case of portable systems or as regards limiting thermal heat-ups in all cases.
Finally, the manufacturing uncertainties, owing to the typical dimensions of the deformable element (the ratio of the bending deflection of the deformable element to its length may be from 1 to 200), is a contributory factor in reducing contact reliability.